For 2021 - online Python 3 training - see ((here)).
Our plans were to retire in summer 2020 and see the world, but Coronavirus has lead us into a lot of lockdown programming in Python 3 and PHP 7.
We can now offer tailored online training - small groups, real tutors - works really well for groups of 4 to 14 delegates. Anywhere in the world; course language English.
Please ask about private 'maintenance' training for Python 2, Tcl, Perl, PHP, Lua, etc.
I asked who I was logged in as yesterday ... and I got two different answers:
[melksham ~]# who am i
trainee pts/1 2009-07-21 13:16 (192.168.200.80)
[melksham ~]# whoami
When you log in to a Unix or a Linux box, you give an account name (and password, I hope!) and you're given a user account name / identity. Or so you think - but really you have two identities, your real
and your effective
id. If you use a command such as su
to get a new identity, your effective id changes, but not your real id
... and that's what had happened with the report above.
a good reason for this ... the difference between real and effective ids is used within programs too - operating system programs such as the passwd
program. On one hand, users cannot possibly be allowed to write from their normal accounts to the file that contains encoded passwords - think of the security risk, yet on the other hand they must
write to that file if they're going to be changing their password. The conundrum is neatly overcome by having the passwd
program set up with a setuid
bit, which means that while you're running it, you have an effective root id while your real id remains as the user you logged in as.
We tell you more about su
and why you should always run su -
on our Linux Admin Introduction
. (written 2009-07-22, updated 2009-07-23)
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articlesA161 - Web Application Deployment - Users and Groups 
Functions and commands with dangerous names - (2005-08-11) 
File permissions of Linux and Unix systems - (2005-08-31) 
Supporting users on Linux and Unix - (2006-04-13) 
Setting up a new user - Linux or Unix - (2008-03-26) 
User and Group settings for Apache httpd web server - (2008-04-22) 
Looking for files with certain characteristics (Linux / Unix) - (2008-05-22) 
The Longest Possible Day - (2008-08-26) 
sstrwxrwxrwx - Unix and Linux file permissions - (2008-11-23) 
Ruby, Perl, Linux, MySQL - some training notes - (2008-11-23) 
Ask the Tutor - Open Source forum - (2009-03-25) 
Choosing a railway station fairly in PHP - (2009-04-04) 
Always use su with minus. And where do programs come from? - (2009-05-27) 
Root is root for a reason! - (2009-11-03) 
su or su - ... what is the difference? - (2010-02-17) 
Linux Web Server - User Roles, User Accounts, and shared administration - (2013-03-16)
Some other Articles
Hot Tap at West Wilts ShowExtracting real data from an exported file in PHP or PerlTurning potential customers awaySee us at the West Wilts ShowMistaken identity?What does x on a linux directory mean?How much space does my directory take - LinuxMelksham Carnival Parade - the peopleStanding on the corner, Melksham CarnivalVariable scope - what is it, and how does it Ruby?
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This is a page archived from The Horse's Mouth at
the diary and writings of Graham Ellis.
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